Disjunction introduction or addition (also called or introduction)[1][2][3] is a rule of inference of propositional logic and almost every other deduction system.

The rule makes it possible to introduce disjunctions to logical proofs.

More generally it's also a simple valid argument form, this means that if the premise is true, then the conclusion is also true as any rule of inference should be, and an immediate inference, as it has a single proposition in its premises.

Disjunction introduction is not a rule in some paraconsistent logics because in combination with other rules of logic, it leads to explosion (i.e. everything becomes provable) and paraconsistent logic tries to avoid explosion and to be able to reason with contradictions.

The disjunction introduction rule may be written in sequent notation: where